The present invention relates to variable speed transmission units.
In automobile engineering and for other uses manual gear boxes are being superseded by automatic gear boxes. Essentially, automatic gear boxes in common use are of two types namely, a hydraulic type having a fluid flywheel incorporating a torque convertor and a number of fixed gears to give a finite number of gear ratios between the power input and power output, and what are true continuously variable transmission units incorporating a variator or composed entirely of a variator which allows an infinite number (in practical terms), of gear ratios, within the overall ratio spread of the variator between the input shaft and the output shaft. One example of such a variator transmits torque between two driving discs or pulleys having a driving surface of generally V-shaped cross-section. The belt acts as a torque transmitting element and the pulley diameters are varied to accommodate the speed variations. The variator can also be a hydraulic motor pump system or an electrical motor generator set or the thyroidal traction type, or simply the traction type in which power is transmitted by transferring axial pressure on driving discs. These variators are being used more extensively in motor cars. However, they suffer from certain disadvantages as indeed, do all automatic transmission units. These units are, generally speaking, inefficient in that there is considerable power loss in use and they are of necessity more complex than a manual gear box. Further, these units are relatively heavy, further reducing the power efficiency of the unit, particularly an automobile. The power loss in these variators is of the order of 10% to 20% of the power input.
Power split CVTs (Continuously Variable Transmissions) have been provided in the past. In general, they consist of an input shaft driving a differential gear which divides the drive into two outputs. One output goes direct to the output, while the other goes through a variator, either to the input or output. In this way, the power is split depending on the gear ratio of the differential so that the variator is only called upon to handle a proportion of the total power.
Attempts to develop commercial versions of the above devices have not so far been successful. Power split CVT's which feed the output of the variator to the main output shaft require a very wide ratio spread because of the considerable difference in speeds between the output gear of the differential to the variator and the main output shaft. Such devices have been successfully developed using hydraulic motor-pump type variators but these are too inefficient to be used in modern road vehicles.
Devices where the power is fed back to the input suffer from problems caused by re-circulating torques, and do not provide engine braking. This is discussed in more detail below.
Specifically, variable speed transmission assemblies have been proposed comprising a main power input shaft, a main power output shaft, a variator and a differential gear assembly of the type comprising, an input differential gear unit having an input shaft and a pair of interconnected differential output gear units each having an output shaft in which the speed of the output gear units are linked and are each variable from stationary to a maximum speed relative to the input gear speed with the other gear stationary, the speed of one output gear unit controlling the speed of the other output gear unit for a specific input gear speed. The differential gear assembly is used as a torque splitter delivering an output from one of the differential gear units directly to the main power output shaft and from its other differential gear unit output shaft to the variator. The output of the variator is fed either directly to the main power input shaft or to the main power output shaft. The use of such a power split transmission is well known for reducing the total power handled by the variator. One of the problems is that heretofore, suitable variators have not been available. While it is possible to use hydraulic type variators most of the mechanical type variators such as those using pulleys have such low ratio spread that they are not suitable for use with such power split transmissions.
If the variator feeds directly to the main power output shaft, then a problem arises. At low starting speed the differential gear assembly will undoubtedly split the torque between the variator and the output shaft. Unfortunately, because the main power output shaft is at a low speed then the differential output gear connected to the main power output shaft is at a relatively low speed relative to the other output shaft of the differential output gear unit. Accordingly, the input speed to the variator from this other differential output gear unit is high. Thus, the input speed to the variator is relatively high while the variator has to feed the main power output shaft so that output speed is low. This means that there is a relatively wide spread between input and output of the variator at start-up and at low speeds. Unfortunately, practical variators such as the pulley and traction types cannot handle the ratio spread required and complex arrangements such as hydraulic systems having a wide ratio spread are required.
If the alternative arrangement is considered namely, the variator feeding the input shaft it can be quite easily demonstrated that there is a certain amount of recirculation of power between the variator, main power input shaft and differential gear assembly so that part of the advantage of the power split is lost with resultant increased friction losses. A further problem is that in normal running, because all three differential gear units of the differential gear assembly are rotating in the same direction when the planet carrier is the input element and thus, on overrun i.e. when there is a tendency for the main power output shaft to run at a speed higher than that matched by the main power input shaft there is a reversal of the torque to the variator because the annulus becomes the input element into the system, which produces an enormous mechanical strain on the variator. Accordingly, these units require the use of free wheels or elaborate gearing arrangements.
In this specification the term "connected" means a direct physical connection, stepped up or down as necessary by gearing, while "fed" is the term used where the connection may not be direct but through another component of the transmission. For example, the main power input shaft feeds the main power output shaft but is not connected to it.